The present invention relates generally to optimization of communication equipment with respect to size and undesired signal interference. More particularly the invention relates to a ceramic feedthrough interconnection assembly. The invention also relates to an optoelectrical capsule and an optoelectrical transceiver according to the preamble of claim 11.
Very simple electronic components and elements, such as resistors, capacitors and inductors, may be placed directly onto a circuit board. However, more complex units, such as integrated circuits and optoelectrical components, are most commonly encapsulated or by other means arranged within a protecting package before being attached to the circuit board. In the latter case, a feedthrough of one or more electrical conductors is necessary in order to accomplish an electrical contact between the unit inside the capsule and relevant circuitry outside the same.
When processing information signals of a comparatively high frequency or bitrate it is generally preferable to shield these signals to the utmost possible extent from any other signals in order to reduce the risk of unintended interference between the high-frequency information signals and the other signals (possibly also of high frequency). Therefore, a capsule containing a high-frequency communication unit is in most cases electrically shielded. Consequently, a lead feedthrough for this kind of capsule must also be electrically shielded. Moreover, in order to reduce the risk of signal reflections (in turn resulting in e.g. distortion), the feedthrough should have a characteristic impedance with respect to the electrical information signal, which matches the impedance that this signal experiences otherwise.
Canadian patent No. 2,305,954 discloses a package for a high-frequency device. A metallic casing here surrounds a photodetector. Metallic signal terminals pass through the casing via a glass insulator. Moreover, each terminal is flanked by a pair of conductive protruding portions that are formed on a sidewall of the casing. The protruding portions have a design, which is intended to match the characteristic impedance for the signal being transported via the terminal. However, since the same terminal is surrounded by first ambient air (on the outside of the casing), second glass (in the casing wall) and third again a gas, e.g. air (inside the casing), the impedance match cannot be altogether optimal. On the contrary, the different dielectric constants of the different surrounding materials will inevitably cause impedance mismatches with respect to the signal sent via the terminal and therefore cause signal reflections.
U.S. Pat. No. 4,873,566 describes a multilayer ceramic laser package to which an unshielded high bitrate input signal is fed on a differential signal format via a first face of the package. A set of auxiliary electrical conductors for communication of various types of relatively low-frequency signals are fed through a second face of the package. Finally, an optical output signal is transmitted over an optical fiber being attached to a third face of the package. The document also suggests that the dimensions of the metallized areas per se, which constitute the conductor surfaces, should be adapted such that the impedance of the high frequency input can be better matched to the optical device.
Methods for shielding a conductor, which is fed through a ceramic capsule wall are known per se, for example via U.S. Pat. No. 4,922,325. This document discloses a multilayer ceramic package, which simulates the performance of a conventional coaxial connector with respect to a high-frequency information signal. Thus a high-quality transmission of this signal through the ceramic capsule wall is made possible.
Today, there is a strong market demand for communication units having as small size as possible. Therefore, it is interesting to concentrate the number of processed information bits per physical volume unit as much as possible and thus reduce the overall size of the equipment. For the same reason, capsules containing sub-units of a particular communication unit should also be placed as close as possible to each other.
One way to economize the circuit board area in a communication unit is to reduce each capsule's footprint on the circuit board. This can be accomplished by positioning the capsules such that each capsule has a projection on the circuit board which is smaller than the capsule's largest side. Unfortunately, this strategy is prone to cause other problems. Namely, positioning a capsule with a non-largest side towards the circuit board severely limits the width of the direct physical interface between the capsule and the circuit board. Hence, in order to maintain an acceptable lead density with respect to the risk of signal interference between neighboring electrical leads to the capsule, the leads would need to be distributed also over an area of the capsule which is not immediately proximate to the circuit board. However, this would in turn again increase the risk of signal interference due a general prolongation of the leads between the capsule and the circuit board. Furthermore, the assemblage of such a capsule would become relatively complicated. For example, the package design proposed in U.S. Pat. No. A, 4,873,566 would either require relatively long leads for transporting the high-frequency signal between the circuit board and the first face of the package or require a corresponding set of relatively long leads for transporting the low-frequency signals between the second face of the capsule and the circuit board.